Milk Analysis

An accurate analysis of milk requires much skill and the equipment of a well-organised laboratory, but it is often important for physicians to be able to judge for themselves of the quality of food so universally prescribed by them, and it is therefore deemed appropriate to describe some of the easier methods of qualitative examination. The minimum standard to which milk must conform, as established by the New York City Health Board, is as follows:

Reaction, Colour, Taste, Odour

Good cow's milk should be almost neutral, reddening blue litmus paper but very slightly if any. The normal colour is white and is due to the fat globules, but it may be slightly yellow, especially if the cow has been feeding in wheat fields (Hind). The taste is sweet and the odour faint and fresh. Bad milk often has a bad odour, either sour or derived from absorption from some neighbouring material. It is sour to the taste, reddens litmus paper strongly, and if held to the light in a test tube or small thin glass it may have a bluish or reddish tinge and appear watery. It curdles in tough, stringy, or glutinous yellowish lumps of large size.

Estimation Of Total Solids

To estimate the total solids of milk several tests are employed. Their object is to furnish data in regard to the nutrient power of the milk and to detect adulteration.

A common method used by health boards is to weigh and evaporate five cubic centimetres of milk to dryness in a small, flat-bottomed, platinum crucible of a known weight. This process takes about an hour. The residue is then dried for an hour more at 100° F. and weighed. The milk dries in two layers. The upper layer consists of a thin film of fat, which is dissolved in petroleum and benzine. The lower honeycombed layer is then washed and dried and weighed again. The loss in weight after removing the upper layer represents the amount of fat present. The soluble salts and soluble proteid are then dissolved out by warm water and alcohol and separated. Ignition over a Bunsen burner drives off all organic matter, and leaves the ash, which is again weighed.

Another less laborious method consists in the use of an instrument called a "lactometer," by which the specific gravity of the milk is taken.

A lactometer is a glass spindle resembling a urinometer, but is about fifteen centimetres long, and it easily floats in a quart measure. It is graduated to measure the specific gravity from o° (the water line) to 400. Milk in which it floats at below 290 is almost surely watered, or if it floats above 330 the milk is skimmed. The specific gravity of the milk being taken with the lactometer, the amount of dilution is readily computed by comparing it with the normal standard, 300. Suppose this specific gravity to be 210 in a given case, then 30: 21:: 100: 70. That is, the normal gravity, 30, is to the gravity of the sample examined as 100 parts of normal milk are to the sample, or, in other words, only 70 parts per hundred of the sample are milk, and 30 parts of water have been added. To facilitate the calculation, the space between 0° and 400 may be divided into 100 parts; each division of the second scale represents 1° or 1 per cent of milk. Thus, if the lactometer stands at 500 of this second scale, there is only 50 per cent of milk in the sample.

There has been much dispute regarding the use of the lactometer, on the ground that an excessive amount of cream, on account of the lightness of the fat, diminishes the specific gravity of a really rich milk. On the other hand, it is claimed that the diminished gravity is always due to increase in the quantity of water, and that milk rich in cream will also contain less water, and therefore maintain the normal standard. The matter has been tested in the courts, and in New York city the lactometer test alone is always sufficient to insure conviction in trials for adulteration of milk with water.

Sometimes in estimating the specific gravity acetic acid is first added to precipitate the casein, which is removed with the fat; the specific gravity of the whey is then taken. Hassell claims that the latter is more constant that the specific gravity of milk itself.